1. Field of the Invention
This invention relates to sensing apparatus and, more particularly, to methods and apparatus for sensing a particular piece of electronic equipment which may be attached to a computer system capable of operating with many different types of equipment.
2. History of the Prior Art
Personal computers are designed to be used by persons of varying capabilities. For example, they may be used persons having no background in computer operation or theory whatsoever or by persons who themselves design computers. One way to make a single type of computer appeal to persons having a very broad spectrum of backgrounds is to design the computer to hide from the operator many of the technical details necessary to the operation by performing those details automatically. The line of MacIntosh personal computers manufactured by Apple Computer Company, Cupertino, Calif., emphasizes this philosophy in its design.
One way that this is accomplished in certain of the MacIntosh computers is to make the computers able to determine the various pieces of peripheral equipment connected to the central processing unit so that the operator need not do this during system set up. For example, the bandwidth, number of lines, numbers of pixels per line, height, and width are among the characteristics which vary from monitor to monitor. It is necessary for the central processing unit to know the characteristics of the particular output display monitor to which it is connected in order to present the information correctly on the display monitor. One method of automatically telling the central processing unit that a particular video output monitor is attached is to provide a three wire connector between the main body of the computer and the video output monitor. The three wires allow eight distinct binary numbers to be sensed by the central processing unit, each of which is used to indicate a different monitor.
However, with the growth of this particular line of computers, it has become apparent that many more than eight monitors may be used with these computers. Consequently, provision must be made for indicating to the central processing unit that one of a much larger number of monitors is connected to provide output for the system. Moreover, because the three wire system is already implemented for a large number of systems in use, it is very desirable that the three wire system continue to function in the same manner to indicate that one of the already designated output monitors is connected yet provide the additional facility to indicate the presence of many additional monitors. In addition, the lack of more pin-out terminals on particular pieces of equipment already set to provide three terminals for such a detection system, emphasizes the need to maintain the three wire design.